1. Field of the Invention
The present invention relates to liquid-crystal display (LCD) apparatuses, and more particularly, to an active-matrix liquid-crystal display apparatus using time-division driving.
2. Description of the Related Art
Active-matrix liquid-crystal display (LCD) apparatuses have been mainly used in personal computers and word processing units. Active-matrix LCD apparatuses are superior in terms of response speed and image quality, and are best suited to in improving recent color display. In such a display apparatus, a nonlinear device such as a transistor and diode is used in each pixel of the LCD panel. Specifically, thin film transistors (TFTs) are formed on a transparent insulating substrate such as a glass substrate.
For active-matrix LCD apparatuses, it is said that a so-called dot-inversion driving method, in which the polarities of voltages to be applied to adjacent dots (pixels) are inverted, is good in improving image quality. This is because inverting the polarities of voltages to be applied to adjacent dots cancels potentials jumped from signal lines, which are caused by capacitors formed at the crossover points of signal lines and gate lines, and thereby stable pixel potentials are input to reduce flickers in LCD display.
On the other hand, if the dot-inversion driving method is not employed, the ground level of gate lines fluctuates and thereby the gate switches of TFTs cannot hold off states. Consequently, held pixel potentials are discharged. The transmission factors of pixels decrease and their contrast is also reduced. In addition, since potentials having the same polarity are jumped from signal lines, pixel contrast becomes conspicuous between alternate lines. Even if an image having the same graduation is displayed, different graduations are shown in alternate lines.
Since the dot-inversion driving method solves these inconveniences, it is an effective driving method for LCD apparatuses to improve image quality.
The outputs of an external driver IC for driving an LCD panel usually correspond to the signal lines of the LCD panel in a one-to-one correspondence relationship. In other words, each output of the driver IC is sent to the corresponding signal line. On the other hand, to make the driver IC compact, there has been known a so-called time-division driving method as an LCD-panel driving method which allows the number of the output pins (output terminals) of the driver IC to be reduced.
In this time-division driving method, a plurality of signal lines are handled as one block, a driver IC outputs a signal to a plurality of signal lines in one division block in a time sequential manner, time-division switches are provided for an LCD panel in units of division blocks, and the time-sequential signal output from the driver IC is time-divided by the time-division switches and sequentially sent to a plurality of signal lines.
When time-division driving is applied to a general driver IC used for dot-inversion driving, however, since the output signals of the driver IC used for dot-inversion driving change their polarities between odd-numbered lines and even-numbered lines, it may occur that dot-inversion driving cannot be used in time-division driving. With divided-by-two time-division driving being taken as an example, this issue will be described below.
As an example of divided-by-two time-division driving, a system shown in FIG. 17 is formed such that two adjacent signal lines 71-1 and 71-2, 71-3 and 71-4, . . . are handled as blocks irrespective of the corresponding colors, red (R), green (G), and blue (B), and the time-division switches 72-1 and 72-2, 72-3 and 72-4, . . . connected to these signal lines divide in time time-sequential signals sent through output lines 73-1, 73-2, . . . from a not-shown driver IC and sequentially send to the signal lines 71-1 and 71-2, 71-3 and 71-4, . . .
In divided-by-two time-division driving in the system configured as described above, since signal voltages inverted in polarity between odd-numbered and even-numbered output terminals of the driver IC are distributed to odd-numbered and even-numbered actual pixels and their polarities are inverted in alternate lines, it is clear from FIG. 18, which shows signal-voltage write conditions, that the polarities of the voltages applied to adjacent pixels in one line cannot be inverted in the entire pixel area, namely, dot inversion cannot be achieved in the entire pixel area.
In FIG. 18, the horizontal direction indicates a scanning order and the vertical direction indicates the order in which the time-division switches operate. A high-voltage write condition is indicated by H, and a low-voltage write condition is indicated by L.
As another example of divided-by-two time-division driving, a system shown in FIG. 19 is formed such that two adjacent signal lines 81-1 and 81-4, 81-2 and 81-5, 81-3 and 81-6, . . . for each color of R, G, and B are handled as blocks, and the time-division switches 82-1 and 82-4, 82-2 and 82-5, 82-3 and 82-6, . . . connected to these signal lines divide in time time-sequential signals sent through output lines 83-1, 83-2, . . . from a not-shown driver IC and sequentially send to the signal lines 81-1 and 81-4, 81-2 and 81-5, 81-3 and 81-6, . . .
In divided-by-two time-division driving in the system configured as described above, since signal voltages inverted in polarity between odd-numbered and even-numbered output terminals of the driver IC are distributed to odd-numbered and even-numbered actual pixels and their polarities are inverted in alternate lines, it is clear from FIG. 20, which shows signal-voltage write conditions, that dot inversion cannot be achieved at boundaries of division blocks in one line. Since the definition of dot inversion does not cover the case which happened at the boundaries of the division blocks, pixel potentials fluctuate and vertical lines appear.
In FIG. 20, the horizontal direction indicates a scanning order and the vertical direction indicates the order in which the time-division switches operate. A high-voltage write condition is indicated by H, and a low-voltage write condition is indicated by L.
In other words, when a time-division number is even, the polarity of the signal voltage A first written in a division block is opposite that of the signal voltage B last written in FIGS. 18 and 20. Since signal voltages sent from the driver IC are inverted between odd-numbered dots and even-numbered dots, signal voltages B1, B2, . . . written last in division blocks have the same polarities as signal voltages A2, A3, . . . written first in the following division blocks.
Therefore, in the first example of divided-by-two time-division driving, dot inversion cannot be achieved in the entire pixel area, and in the second example of divided-by-two time-division driving, dot inversion cannot be achieved at the boundaries of division blocks. Hence, image quality is reduced. Polarity inversion, however, can be achieved with chroma signals being rotated. As will be described later, this makes data re-arrangement processing complicated and increases the size of a processing circuit.
The present invention has been made in consideration of the above inconveniences. Accordingly, it is an object of the present invention to provide an LCD apparatus which allows time-division driving to be implemented without reducing image quality.
The foregoing object is achieved according to the present invention through the provision of a liquid-crystal display apparatus including: a display section formed of a plurality of row gate lines, a plurality of column signal lines, and a plurality of pixels two-dimensionally arranged at the intersections of the plurality of row gate lines and the plurality of column signal lines; a transparent substrate on which the display section is formed; a second transparent substrate having an opposite electrode, connected to the transparent substrate with a predetermined gap placed therebetween; liquid crystal held in the gap; a driver circuit for outputting a time-sequential signal corresponding to a predetermined time-division number; and a time-division switch for time-dividing the time-sequential signal output from the driver circuit and for sending them to the corresponding signal lines among the plurality of column signal lines, wherein the time-division number used in the time-division switch is set odd.
In the LCD apparatus having the above configuration, the driver circuit outputs a time-sequential signal corresponding to the time division number to allow time-division driving. The time-sequential signals, for example, in dot-inversion driving, are signals having different polarities alternately (dot-inversion signals). Time-division switches apply time division to the time-sequential signals with an odd time-division number and send them to the corresponding signal lines. With this operation, the voltages applied to adjacent pixels in one line do not have the same polarity, and dot inversion driving is achieved in the entire pixel area.